One way automatic meter reading system

ABSTRACT

A meter interface unit for use in a one way automatic meter reading system comprises first means for frequently transmitting a short burst transmission suitable for reception by a hand held or mobile receiver, and second means for transmitting narrow band data messages suitable for reception by a fixed network first tier receiving station. The receiver at the first tier receiving station has means for suppressing the short transmissions to prevent interference with the slow busts from the second transmitting means. Repeated packet collisions between the short bursts are avoided by randomly varying the time period between the short bursts.

CROSS REFERENCE TO RELATED APPLICATIONS

Applicants claim priority under 35 U.S.C. §119 of Great BritainApplication No. GB 0005265.4, filed on Mar. 6, 2000. Applicants alsoclaim priority under 35 U.S.C. §365 of PCT/GB01/00873, filed on Feb. 28,2001. The international application under PCT article 21(2) waspublished in English.

BACKGROUND OF THE INVENTION

The present invention relates to automatic meter reading (AMR) and, morespecifically, to a meter interface unit for use in a one way automaticmeter reading system. In such a “one-way” system, as opposed to an“on-demand” system, the meter interface unit transmits only. Thereforethe cost of the unit is kept to a minimum by reducing the necessaryradio circuitry by the elimination of the receiver required by on-demandsystems.

There are two types of automatic meter reading system architecture,“mobile” and “fixed” network systems. In the mobile type the meters areread by hand-held or mobile readers which travel through the vicinity ofthe meters to be read. In this case, a radio transmitter which has onlya relatively short range is needed. In the second fixed network type ofautomatic meter reading system, a fixed network is provided to whicheach meter interface unit transmits. In this case, the transmitter inthe meter interface unit requires a relatively long range if the numberof receiving stations in the fixed network is to be kept the reasonablenumber.

1. Technical Problem

In many circumstances where a fixed network is to be installed there isa technical problem in finding suitable sites for the first tier ofreceiving stations.

2. Prior Art Solutions

This can be overcome by using hand-held or mobile readers for this firsttier. GB-A-2330279 (WO 99/18700) describes a way of generating andreceiving a short low-energy transmission that can be made veryfrequently, allowing the messages to be received with a minimum ofdelay. If the transmissions can be made sufficiently often and the radiorange is adequate, a vehicle can travel at a normal road speed of, say,40 km/h and still pick up all the readings.

Disadvantages of the Prior Art

Unfortunately because of the restriction on the battery power availableand in order to minimise channel congestion these “blind” transmissionsmust be very short and at high data rates. This implies a wide radioband width and consequential low sensitivity for the reader's receiverhence giving a restricted range. If the same fast frequent transmissionsare used in a fixed network system the range will be verylimited,.requiring a very high density of receiving points.

Installation of the meter interface units is one of the set up costs ofinstalling an automatic meter reading system. Clearly it is notpractical to replace the meter interface unit after installation if alocal site for a first tier receiving station is found.

Example of One Way Fixed Network Requirements

In an average low power mobile AMR system a range of about 100 m can beachieved between the MIU and the reader. This corresponds to a schemewith a link budget of about 110 dB (10 dBm transmitter and a −100 dBmsensitivity receiver) and an operating frequency of 400–1000 MHz. If thereading vehicle is travelling at about 40 km/h the maximum time betweenthe transmissions is limited to about 5 seconds. This ensures that thereader has the opportunity to receive at least two transmissions fromeach MIU. The duration of the data message will be in the range of 1–2mS and if the message is about 120 bits long a data rate of about 100kbps is needed. If the modulation used is Direct Frequency Shift Keying(DFSK) the receiver noise bandwidth must be approximately 400 kHz.

In a purely fixed network system the MIU is always within range of thereader and so the rate of transmissions is less important. A few timesper day might be adequate with a typical rate of once every 4 hours.This means that the transmissions could last much longer with the samebattery capacity and so the message could be sent at a slow data rate ofsay 200 bits/second. If the same data packet and modulation index isused the receiver noise bandwidth could be reduced to about 4 kHz. Thiscould allow an improvement in the receiver sensitivity of about 20 dB.Elevating the receiver antenna and providing a modest amount of antennagain could give a further 10 dB of link budget.

The total gain over the fast data rate mobile system is therefore 30 dBor 1000× power gain. In theory with free space propagation this wouldgive a √1000 or 333 fold increase in the 100 m range. In practice withaverage urban conditions the improvement will be 7 to 20 times. Thiswould mean that a 100 meter range wide band mobile system would achieveabout 700 meters to a narrow band fixed receiver.

SOLUTION OF THE INVENTION

In accordance with the present invention there is provided a meterinterface unit comprising means for connection to a utility meter whichis to be read in a one way automatic meter reading system, wherein theinterface unit is not provided with any means for receiving signals andcomprises first means for frequently transmitting a short bursttransmission suitable for reception by a hand held or mobile receiver,characterised in that the interface unit further comprises second meansfor transmitting narrow band data messages suitable for reception by afixed network first tier receiving station.

In the following description the transmissions from the firsttransmitting means will be described as fast packets and thetransmissions from the second transmitting means will be described asslow packets.

Such a solution has the advantage that when sites for first tierreceiving stations are found these can be brought into use immediatelywithout any change to the operation of the associated meter interfaceunits. The fast packets are simply ignored and the slow packets decodedinstead. This allows simple upgrade of a mobile system to a fixednetwork system or alternatively reading of meters from a vehicle readerif a fault develops with the fixed network. The presence of twotransmitting means in the same meter interface unit creates its owntechnical problems but it has been found that these can be overcome inthe embodiments described.

In accordance with the invention there are also provided receivers foruse with such meter interface units that can eliminate the interferencefrom the unwanted transmission.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be well understood some embodimentsthereof will now be described, by way of example only, with reference tothe accompanying diagrammatic drawings, in which:

FIG. 1 shows a diagrammatic representation of a receiver in accordancewith a first embodiment of the present invention for use in a slow modefixed network first tier receiving station;

FIG. 2 shows a diagrammatic representation of an alternative embodimentof a slow mode receiver; and

FIG. 3 a diagram illustrating the timing of fast packets in order toavoid data collision.

DESCRIPTION OF PREFERRED EMBODIMENTS

A meter interface unit (MIU) includes a first slow transmitter and asecond fast transmitter. Both transmitters are capable of operatingsimultaneously in order to send both fast and slow data packets. Thesimplest solution would be to use a different radio channel for eachtype of transmission, but this would add considerable cost to thetransmitter in the MIU. In the embodiment described, both transmittersuse the same channel and there is a single modulator which generatesboth the fast and slow packets.

If the MIU modulator has sufficient bandwidth, the slow mode 200 bpsdata transmissions can be interleaved with fast mode 100 kbps datatransmissions. For example the fast mode can be sent every 5 second andthe slow mode every 8 hours. It may be necessary to reduce the rate ofthe fast transmissions to have sufficient battery power available forthe slow transmissions.

A mobile reader designed to receive the fast packet transmissions is notgoing to be badly affected by the slow transmissions because typicallythey only occur for one second every 8 hours. Even in a situation wherethere are 50 MIUs in the range, the channel will be occupied with slowtransmissions for less than 0 2%.

If we assume that the fast transmissions from all the MIUs in the rangeconfirm to a random pattern with a Poisson type distribution, thetraffic occupancy or through put of the channel R is given by:R=λτ  (2.1)

-   -   Where: λ is the mean arrival rate of the packets per second    -   τ is the packet duration in seconds

The Poisson distributed probability of no transmissions occurring duringany time period is given by:Pr(0)=−e ^(−R)  (2.2)

In a practical example if the 50 fast transmissions from the 50 MIUsoccur every 5 seconds and the slow packet duration is 1 second fromequation (2.1) R=10 and the probability of no interfering fast packetsduring a slow transmission is:

-   -   =1−Pr(0)    -   −1−−e⁻¹⁰    -   =1 approximately

ie there is 100% probability of a slow mode packet being interfered withby a fast mode packet.

In practice FM capture effect will help to suppress any fast packetsthat are weaker than the desired slow mode transmissions but the systemwould still not work for weak signals.

Slow Mode Receiver

If the slow mode is sent at 200 bits/s each bit has duration of 5 mSwhich is considerably longer than the entire fast mode burst. Thiscreates the effect of a small hole punched in each data bit by the fasttransmissions. Typical existing slow receivers employs a narrow filterto get the desired noise bandwidth and the effect of this filter will beto lengthen the short fast transmission to several mS due to ringing.

This present application describes two methods that can be used tosuppress the interference from the fast transmissions in a slow modereceiver at a fixed network first tier receiving station.

Noise Blanker Technique

The first method is similar to that employed in a high performancecommunication receivers to stop ignition interference type noises pulsesfrom being lengthened by narrow filters and hence disrupting reception.

A block diagram of the receiver's first and second intermediatefrequency sections (IFs) is shown in in FIG. 1. It is assumed that aconventional RF amplifier and mixer whose bandwidth is very wideprecedes the first IF.

The first IF filter 10 has a several hundred kHz wide bandwidth so thatit can pass the fast and slow data without any distortion or pulselengthening. The signal from the filter 10 is then passed through atransmission gate 12, which is normally held closed. The signal is thenconverted to a low frequency and narrow band second IF by mixer 14 andlocal oscillator 16.

An output of the mixer 14 is fed to a narrow band IF filter 18 and thento a conventional amplifier and FM detector/data recovery circuit 20.When receiving a slow mode transmission an optional automatic frequencycontrol (AFC) signal can be taken from the detector 20 and applied tothe local oscillator 16, in such a way that the AFC will lock onto theincoming signal and remove any tuning errors. In a practical system itmay be necessary to make the AFC voltage sweep so that it will lock onto the preamble of the slow transmission.

A monostable circuit 24 controls the transmission gate 12. A high-speeddetector 26 is connected to the output of the first IF filter 10. Thedetector 26 is adapted to trigger the monostable circuit 24 if a fastmode burst is detected. This will cause the transmission gate 12 to opento prevent the fast burst from reaching the second mixer 14. In this waythe energy from the fast burst is suppressed before it reaches thenarrow filter 18 and so does not cause it to ring.

The period of the monostable 24 is adjusted to cause a minimumdisruption to the wide data bits in the slow mode transmission.Typically, this could be 1–2 mS, which is only a fraction of the 5 mSslow data bit period.

If the fast mode transmission is too weak to trigger the detector 26 itwill probably also not affect the slow data.

DSP Approach to Suppression of Fast Packets

Digital Signal Processing (DSP) offers a simple solution to the receiverfor the slow mode transmissions and this is shown in FIG. 2. Thisreceiver also requires a mixer and local oscillator (not shown) toconvert to the IF frequency. The received signal is then fed to a wideIF filter 30 typically at 10.7 MHz. The filtered signal from 30 is thenamplified by an amplifier 32 and applied to an analogue to digitalconverter (ADC) 34. The digital signal is then processed by a DSP 36,and the recovered slow data passed to the rest of the system.

The DSP software can be used to implement a function similar to thehardware described in relation to the embodiment of FIG. 1.Alternatively, it is possible to synthesise narrow Finite ImpulseResponse (FIR) filters in the DSP that do not ring. In this way therewould be no lengthening of the fast bursts and so they could be removedin the software without affecting the wanted data.

Improved Data Collision Technique

The system described above relies on all the transmissions from each ofthe MIUs being received in a random manner. In practice this is notalways easy to achieve. One major hazard is that if on one cycle twofast packet transmissions collide it is likely that on the next cycle,say 5 seconds later they will collide again. It will then be necessaryto wait until the slight differences between the clocks of the two MIUsallow them to drift sufficiently for the packets not to overlap.

Previously this has been achieved by using low stability RC timed clocksto guarantee sufficient error. It is likely that in future the nextgeneration of MIUs will have internal real time clocks. These will bedesigned to have the best possible stability and so if they are used todetermine the transmission time two conflicting MIUs could stay insynchronisation for a consideration time making it nearly impossible toread either MIU. To overcome this problem it is proposed to take theregular and accurately timed clock period but add some randomisation ofthe timing to avoid consecutive collisions. In this way the firsttransmitter varies the time period between each successive short bursttransmission. This solution is illustrated with reference to FIG. 3.

The basic timing period is defined at T e.g: 4 second and to this aperiod of t×n (where t is the duration of a packet and n is a randomnumber) is added to define the timing of the next fast packet.

The random or pseudo random number n can be generated in a number ofways. For example:

-   1. By using a random number generator such as a pseudo noise (PN)    sequence with the unit serial number as the seed. This could be    implemented in either software or hardware.-   2. Use a fast counter that is reset by a metering pulse so that n is    a function of when the last metering pulse occurred.

It is preferable that the maximum randomising period nt is only afraction of the main period T to prevent very large variations in thetime between transmissions which might be a disadvantage in a mobilesystem. The use of the stable clock and true randomising also makes thecalculation of battery life easier, as the average time betweentransmissions will be exactly T+nt/2.

1. A meter interface unit comprising: means for connection to a utilitymeter which is to be read in a one way automatic meter reading system,wherein the interface unit is not provided with any means for receivingsignals; a first means configured to frequently transmit a short burst(t) transmission suitable for reception by a hand held or mobilereceiver; and a second means configured to transmit narrow band datamessages suitable for reception by a fixed network first tier receivingstation.
 2. A meter interface unit as claimed in claim 1, wherein thefirst and second transmitting means operate on the same radio channel.3. A meter interface unit as claimed in claim 1, wherein the first meansis adapted to vary the time period between each successive short bursttransmission.
 4. A meter interface unit as claimed in claim 3, whereinthe time period is T+n×t where T is a base time period t is the durationof a short burst transmission and n is a random number.
 5. A receiverfor use in a fixed network first tier receiving station for receivingsignals transmitted by the second transmitting means of a meterinterface unit as claimed in claim 1, comprising means configured todetect the presence of a short burst transmission and suppressing thetransmission before the received signal passes through a narrow bandfilter.
 6. A receiver as claimed in claim 5, wherein a high-speeddetector is configured to control a monostable which opens a gate toprevent a short burst transmission reaching a second IF stage of thereceiver.
 7. A receiver as claimed in claim 5, wherein the suppressionis implemented by digital signal processing means.
 8. A meter interfaceunit comprising: a utility meter connection configured to connect to aone way automatic meter reading system; a first transmitter configuredto frequently transmit a short burst (t) transmission suitable forreception by a hand held or mobile receiver; and a second transmitterconfigured to transmit narrow band data messages suitable for receptionby a fixed network first tier receiving station.
 9. A meter interfaceunit as claimed in claim 8, wherein the interface unit is not providedwith any means for receiving signals.
 10. A meter interface unit asclaimed in claim 8, wherein the first and second transmitters operate onthe same radio channel.
 11. A meter interface unit as claimed in claim8, wherein the first transmitter is adapted to vary the time periodbetween each successive short burst transmission.
 12. A meter interfaceunit as claimed in claim 11, wherein the time period is T+n×t where T isa base time period t is the duration of a short burst transmission and nis a random number.
 13. A receiver for use in a fixed network first tierreceiving station for receiving signals transmitted by the secondtransmitter of a meter interface unit as claimed in claim 8, comprising:a detector configured to detect the presence of a short bursttransmission and suppress the detected transmission before the receivedsignal passes through a narrow band filter.
 14. A receiver as claimed inclaim 13, wherein a high-speed detector is configured to control amonostable circuit which opens a gate to prevent a short bursttransmission reaching a second IF stage of the receiver.
 15. A receiveras claimed in claim 13, wherein the suppression is implemented by adigital signal processor.